Atoms, ions, formulae and relative massesEdexcel International A Level Chemistry: Subtopic test

1. 1
   A garden-centre chemist checks the label on a fertiliser that contains ammonium sulfate, (NH₄)₂SO₄. Relative atomic masses: H = 1.0, N = 14.0, O = 16.0, S = 32.1.

   (a)

   What is the relative formula mass of ammonium sulfate?

   [1 mark]

   • A114.1
   • B132.1
   • C124.1
   • D148.1

   (b)

   Why is the term 'relative formula mass' used for ammonium sulfate rather than 'relative molecular mass'?

   [1 mark]

   • AIt contains more than two different elements
   • BIts relative mass has no units, unlike a relative molecular mass
   • CIt is a mixture of ammonium ions and sulfate ions
   • DIt has a giant ionic structure, so there are no discrete molecules

   (c)

   Calculate the percentage by mass of nitrogen in ammonium sulfate. Give your answer to 3 significant figures.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Chemists measure relative atomic masses on the carbon-12 scale. A data book gives the relative atomic mass of magnesium as 24.3, although naturally occurring magnesium contains only atoms with mass numbers 24, 25 and 26.

   (a)

   Which statement gives the correct definition of relative atomic mass?

   [1 mark]

   • AThe weighted mean mass of an atom of an element relative to one twelfth of the mass of an atom of carbon-12
   • BThe mass of one atom of the element relative to the mass of one atom of carbon-12
   • CThe mass in grams of one mole of atoms of the element
   • DThe total mass of the protons and neutrons in the most abundant isotope

   (b)

   Which statement explains why the relative atomic mass of magnesium is not a whole number?

   [1 mark]

   • AElectrons contribute fractions of a unit to the mass of each atom
   • BA neutron has a mass of about 0.3 on the carbon-12 scale
   • CIt is a weighted mean of the masses of its isotopes, allowing for their abundances
   • DMagnesium atoms lose a little mass when they form bonds

   (c)

   Explain why the relative atomic mass of magnesium has no units, whereas the molar mass of magnesium is 24.3 g mol⁻¹.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A school air-quality sensor reports the carbon dioxide concentration in a sealed classroom of volume 180 m³ as 1250 ppm by volume. Dry air contains 0.93% argon by volume. 1 m³ = 1000 dm³.

   (a)

   Calculate the volume, in dm³, of carbon dioxide in the classroom.

   [3 marks]

   (b)

   Calculate the concentration of argon in ppm, the volume of argon in the room in dm³, and how many times greater this volume is than the volume of carbon dioxide.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A teacher sets a revision task on chemical terminology. Part 1: she displays four species: magnesium metal (Mg), a magnesium ion (Mg²⁺), oxygen gas (O₂) and sodium chloride (NaCl). Part 2: she gives data on a compound Y, which has empirical formula CH₂ and relative molecular mass 84.0. Relative atomic masses: H = 1.0, C = 12.0, Na = 23.0, Cl = 35.5.

   (a)

   Explain how the term relative formula mass differs from relative molecular mass, using sodium chloride as an example, and calculate the relative formula mass of sodium chloride. State the units, if any, of the relative formula mass and of the molar mass of sodium chloride.

   [6 marks]

   (b)

   Explain the difference between an empirical formula and a molecular formula, deduce the molecular formula of Y, state its molar mass with units, and show that Y has the same percentage by mass of carbon as ethene, C₂H₄.

   [6 marks]

   Total for question 4: 12 marks

End of questions